Make gradle support SWT on Windows/Linux

This ought to make AppleCommander build properly using gradle on Windows
and Linux, and makes adding other platforms pretty easy too.

build.gradle

@@ -8,6 +8,29 @@ apply plugin: 'application'
 // Define the main class for the application
 mainClassName = 'com.webcodepro.applecommander.ui.AppleCommander'
 
+// Configure build for each supported platform
+def os = System.getProperty("os.name").toLowerCase()
+def arch = System.getProperty("os.arch")
+ext {
+	if (os.contains("windows")) {
+		swtGUI = "win32.win32.$arch"
+	} else if (os.contains("mac os x")) {
+		swtGUI = "cocoa.macosx.$arch"
+	} else if (os.contains("linux")) {
+		swtGUI = "gtk.linux.$arch"
+	} else {
+		throw new GradleException("Don't know how to build for this platform:\n" +
+				"      os = \"$os\"\n" +
+				"    arch = \"$arch\"\n\n" +
+				"Edit build.gradle to add support")
+		/* At least you'll need to define swtGUI for your os and
+		 * probably your arch too.  Most SWT backend GUI descriptions
+		 * take the form of <widgetset>.<os-tag>.<arch>.
+		 */
+	}
+}
+
+
 // In this section you declare where to find the dependencies of your project
 repositories {
     mavenCentral()
@@ -17,8 +40,8 @@ repositories {
 }
 
 dependencies {
-    compile "org.eclipse.swt:org.eclipse.swt.cocoa.macosx.x86_64:$swtVersion"
-    
+    compile "org.eclipse.swt:org.eclipse.swt.$swtGUI:$swtVersion"
+
     compileOnly "org.apache.ant:ant:$antVersion"
 
     testCompile "junit:junit:$junitVersion"
@@ -27,7 +50,7 @@ dependencies {
 jar {
     manifest {
         attributes 'Main-Class': "$mainClassName"
-    } 
+    }
     doFirst {
         from { configurations.runtime.collect { it.isDirectory() ? it : zipTree(it) } }
     }